blender reports non existant tris
This is not a bug, it's a feature! :-D

Joke aside, blender is letting you know how many triangles you would have had you converted all your polygons to triangles.

The tri-count is an important number for lots of purposes, including game asset creation (where you always try to maintain the lowest polycount possible), 3D printing and other areas,

Several file formats support only triangles (STL for instance), and convert everything to tris implicitly (while others, like Collada, support quads too but convert to tris by default). So this gives you a preview of the number of tris you will have after converting.